Service unit for a hub having multiple star interconnect configurations

ABSTRACT

An architecture for a SONET network element, such as a hybrid STM/ATM add-drop multiplexer. The disclosed system includes an interconnection system for a network element, including a line unit slot, a switch fabric slot, and two or more service unit slots. The line unit slot is connected as a hub to the switch fabric slot and the service unit slots in a first star interconnection configuration. The switch fabric slot is connected as a hub to the line unit slot and the service unit slots in a second star interconnection configuration. The star interconnection configurations provide fault isolation between different units, and allow for replacement of failed units without interfering with the links of other units to the hub. In a preferred embodiment, the switch fabric slot and one of the service unit slots comprise the same slot, thus permitting flexible configuration of the device within a minimal space. In a further illustrative embodiment, a control unit slot is provided in the interconnection system, and connected as a hub to the line unit slot, the switch fabric slot, and the service unit slots to form a third star interconnection configuration. A service unit is also disclosed, including a first backplane interface for connecting with an ATM star interconnect configuration within the network element, and a second backplane interface for connecting to an STM star interconnect configuration within said network element. The service unit further includes a third backplane interface to connect with a control star interconnect configuration within the network element.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a Divisional of U.S. patent application Ser.No. 09/325,187, filed Jun. 3, 1999, entitled ARCHITECTURE FOR A HYBRIDSTM/ATM ADD-DROP MULTIPLEXER.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT NOTAPPLICABLE BACKGROUND OF THE INVENTION

[0002] The invention relates generally to communication systems, andmore specifically to an architecture for a hybrid STM/ATM add-dropmultiplexer.

[0003] As it is generally known, SONET (Synchronous Optical Network)defines a set of standards for a synchronous optical hierarchy that hasthe flexibility to transport many digital signals having differentcapacities. A corresponding international synchronous digital hierarchy(SDH) standard provides a set of definitions analogous to those ofSONET. The synchronous nature of SONET is provided by a receive side anda transmit side clock in each network element (NE). In order tosynchronize the receive and transmit clocks, a SONET network element,such as an add-drop multiplexer, includes circuitry to recover clocksignals from various sources that may be available, and to distributehighly accurate clocks internally based on such recovery.

[0004] A central timing source provides a Building Integrated TimeSource, also referred to as a “BITS” clock, that may be providedout-of-band to each network element in a SONET ring. If a networkelement is for some reason not able to receive the BITS clock directly,an embedded clock may be recovered by that device from an incoming linethat should reflect the centrally provided BITS clock.

[0005] The basic building block in SONET is a synchronous transportsignal level-1 (STS-1), which is transported as a 51.840-Mb/s serialtransmission using an optical carrier level-1 (OC-1) optical signal.Higher data rates are transported using SONET by multiplexing N lowerlevel signals together. To this end, SONET defines optical andelectrical signals designated as OC-N (Optical Carrier level-N) andSTS-N (Synchronous transport signal level-N), where OC-N and STS-N havethe same data rate for a given value of N. Accordingly, just as STS-1and OC-1 share a common data rate of 51.84 Mb/s, OC-3/STS-3 both have adata rate of 155.52 Mb/s.

[0006] Information transported via an STS-1 signal is organized asframes, each having 6480 bits (810 bytes). An STS-1 frame includestransport overhead and a Synchronous Payload Envelope (SPE). The SPEincludes a payload, which is typically mapped into the SPE by what isreferred to as path terminating equipment at what is known as the pathlayer of the SONET architecture. Line terminating equipment, such as anOC-N to OC-M multiplexer, is used to place an SPE into a frame, alongwith certain line overhead (LOH) bytes. The LOH bytes provideinformation for line protection and maintenance purposes. The sectionlayer in SONET transports the STS-N frame over a physical medium, suchas optical fiber, and is associated with a number of section overhead(SOH) bytes. The SOH bytes are used for framing, section monitoring, andsection level equipment communication. Finally, a physical layer inSONET transports the bits serially as either electrical or opticalentities.

[0007] The SPE portion of an STS-1 frame is contained within an area ofan STS-1 frame that is typically viewed as a matrix of bytes having 87columns and 9 rows. Two columns of the matrix (30 and 59) contain fixedstuff bytes. Another column contains STS-1 POH. The payload of an SPEmay have its first byte anywhere inside the SPE matrix, and, in fact maymove around in this area between frames. The method by which thestarting payload location is determined is responsive to the contents oftransport overhead bytes in the frame referred to as H1 and H2. H1 andH2 store an offset value referred to as a “pointer”, indicating alocation in the STS-1 frame in which the first payload byte is located.

[0008] The pointer value enables a SONET network element to operate inthe face of certain conditions which may, for example, cause the STS-1frame rate to become faster or slower than the SPE insertion rate. Thissituation may arise when the clock of the NE must be derived from arelatively less accurate clock source, in order to continue operation,when a more accurate source, such as the BITS clock itself, has beenlost. In such a case, an extra byte may need to be transmitted in whatis known as a negative justification opportunity byte, or, one less bytemay be transmitted in a given STS-1 frame so as to accommodate the SPE,thus causing the location of the beginning of the payload to vary.

[0009] Various digital signals, such as those defined in the well-knownDigital Multiplex Hierarchy (DMH), may be included in the SPE payload.The DMH defines signals including DS-0 (referred to as a 64-kb/s timeslot), DS-1 (1.544 Mb/s), and DS-3 (44.736 Mb/s). The SONET standard issufficiently flexible to allow new data rates to be supported, asservices require them. In a common implementation, DS-1s are mapped intovirtual tributaries (VTs), which are in turn multiplexed into an STS-1SPE, and are then multiplexed into an optical carrier-N (OC-N) opticalline rate.

[0010] The payload of a particular SPE may be associated with one offour different sizes of virtual tributaries (VTs). The VTs are VT1.5having a data rate of 1.728 Mb/s, VT2 at 2.304 Mb/s, VT3 at 3.456 Mb/s,and VT6 at 6.912 Mb/s. A superframe consists of four STS-1 frames, andis used to transmit a VT. The alignment of a VT within the bytes of thepayload allocated for that VT is indicated by a pointer contained withintwo VT pointer bytes, which contain a pointer offset similar to theSTS-1 pointer described above.

[0011] Existing add-drop multiplexers (ADMs) are SONET multiplexers thatallow DS-1 and other DMH signals to be added into or dropped from anSTS-1 signal. Traditional ADMs have two bi-directional ports, and may beused in self-healing ring (SHR) network architectures. An SHR uses acollection of network elements including ADMs in a physical closed loopso that each network element is connected with a duplex connectionthrough its ports to two adjacent nodes. Any loss of connection due to asingle failure of a network element or a connection between networkelements may be automatically restored in this topology. Existing ADMshave additionally included a cross-connect matrix for directing STMsignals from one interface to another. Such a cross-connect matrix isreferred to as an STM switch fabric. The manner in which specific STMsignals are directed between interfaces of the STM switch fabric dependson how the network bandwidth has been “provisioned” to the variouscustomers using the network. The path of a signal through a givencross-connect matrix is statically defined based on provisioninginformation provided from a central office or “craft” technician.

[0012] As mentioned above, SONET provides substantial overheadinformation. SONET overhead information is accessed, generated, andprocessed by the equipment which terminates the particular overheadlayer. More specifically, section terminating equipment operates on ninebytes of section overhead, which are used for communications betweenadjacent network elements. Section overhead supports functions such as:performance monitoring (STS-N signal), local orderwire, datacommunication channels (DCC) to carry information for OAM&P, andframing. The section overhead is found in the first three rows ofcolumns 1 through 9 of the SPE.

[0013] Line terminating equipment operates on line overhead, which isused for the STS-N signal between STS-N multiplexers. Line overheadconsists of 18 overhead bytes, and supports functions such as: locatingthe SPE in the frame, multiplexing or concatenating signals, performancemonitoring, automatic protection switching, and line maintenance. Theline overhead is found in rows 4 to 9 of columns 1 through 9 of the SPE.

[0014] Path overhead bytes (POH) are associated with the path layer, andare included in the SPE. Path-level overhead, in the form of either VTpath overhead or STS path overhead, is carried from end-to-end; it isadded to DS1 signals when they are mapped into virtual tributaries andfor STS-1 payloads that travel end-to-end. VT path overhead (VT POH)terminating equipment operates on four evenly distributed VT pathoverhead bytes starting at the first byte of the VT payload, asindicated by the VT payload pointer. VT POH provides communicationbetween the point of creation of an VT SPE and its point of disassembly.

[0015] STS path terminating equipment terminates STS path overhead (STSPOH) consisting of nine evenly distributed bytes starting at the firstbyte of the STS SPE. STS POH provides for communication between thepoint of creation of an STS SPE and its point of disassembly. STS pathoverhead supports functions such as: performance monitoring of the STSSPE, signal labels (the content of the STS SPE, including status ofmapped payloads), path status, and path trace. The path overhead isfound in rows 1 to 9 of the first column of the SPE.

[0016] Asynchronous Transfer Mode (ATM) is a cell-based transport andswitching technology. ATM provides high-capacity transmission of voice,data, and video within telecommunications and computing environments.ATM supports a variety of traffic types, including constant bit-rate(CBR) traffic—like full-motion video and voice —where delays and cellloss cannot be tolerated. ATM also supports variable bit-rate (VBR)applications—like LAN traffic and large file transfers—where delay canbe tolerated.

[0017] ATM establishes virtual connections which may be shared bymultiple users. Each ATM virtual connection is identified by acombination of a Virtual Channel Identifier and a Virtual PathIdentifier, referred to as a VCI/VPI value. ATM is a transporttechnology that formats all information content carried by the networkinto 53-byte cells. Since these cells are short in length and standardin size, they can be switched through network elements known as ATMswitches with little delay, using what is referred to as an ATM switchfabric. Since various types of traffic can be carried on the samenetwork, bandwidth utilization can be very high. These characteristicsmake the network very flexible and cost effective.

[0018] An ATM switch fabric operates to direct ATM cells from oneinterface to another. For a given received cell, the specific outputinterface of the ATM switch fabric is determined in response to aVCI/VPI value contained within the cell. Virtual channel and virtualpath routing information is dynamically modified in the switch fabric asconnections are established and torn down in the network. In this waythe ATM switch fabric operates in response to dynamically changeablevirtual connection information.

[0019] ATM cells may be encapsulated and transmitted over SONET forexample using STS-1 or STS-3c, which is a concatenation of three STS-1signals. STS-1 transports may generally be concatenated, and thecombination then referred to as STS-Nc, where N is the number of STS-1signals that are combined. In the case of STS-3c, the SPE of theresultant STS-3c frame consists of 3×783 bytes, together with POH. Theconcatenated STS-1s are multiplexed, switched, and transported as asingle unit. An overhead byte of the STS-3c frame transport overhead,referred to as the H4 byte, contains an offset indicating the number ofbytes between the H4 byte and the first ATM cell that is contained inthe SPE.

[0020] In many cases customers require support for both ATM switchingand STM switching in their communications systems. However, devicesprovided by vendors to support SONET have typically lacked thecapability to also support ATM. In particular, typical existing ADMshave supported only SONET rings, while existing ATM switches havegenerally supported only ATM. Accordingly, if a customer has needed bothSONET and ATM networks, they have necessarily had to purchase dedicatedSONET equipment (ADMs), in addition to ATM switches. This is costly interms of necessitating multiple devices. In addition, most customerscannot predict what their future communications requirements will bewhen they buy one piece of equipment. Because existing systems have beenrestricted to supporting only one of either SONET or ATM switching, theyhave not been flexible or scalable with regard to adding support for theother protocol. As a result of such inflexibility, changes in customerrequirements may require the purchase of completely new devices tosupport a previously unsupported protocol.

[0021] Accordingly, there is a need for a communication device whichcombines the functions of a SONET add-drop multiplexer with thefunctions of an ATM switch. The device should be capable of multipleconfigurations to support STM only, ATM only, or hybrid STM/ATMoperation. Moreover, the device should be scalable such that additionalfunctionality may be conveniently added as the needs of the customerchange over time.

BRIEF SUMMARY OF THE INVENTION

[0022] An architecture for a hybrid STM/ATM add-drop multiplexer isdisclosed. The disclosed architecture includes an interconnection systemfor a network element, having at least one line unit slot, a switchfabric slot, and two or more service unit slots. The line unit slot isconnected as a hub to the switch fabric slot and the service unit slotsin a first star interconnection configuration. The switch fabric slot isconnected as a hub to the line unit slot and the service unit slots in asecond star interconnection configuration. In a preferred embodiment,the switch fabric slot and one of the service unit slots comprise thesame slot, thus permitting flexible configuration of the device. Tosupport a configuration providing non-STM switching, the switch fabricslot is operable to receive a switch fabric unit that includes a non-STMswitch fabric.

[0023] In an illustrative embodiment, a control unit slot is provided inthe interconnection system, and connected as a hub to the line unitslot, the switch fabric slot, and the service unit slots to form a thirdstar interconnection configuration. Each star interconnectionconfiguration for example consists of dedicated point to pointconnections between the hub and each other slot in the configuration.The point to point connections employ a low voltage, complementarysignaling mechanism, such as Low Voltage Differential Signaling (LVDS),in order to achieve high speeds, while controlling electromagneticinterference (EMI). Redundant line unit and switch fabric slots areprovided, as well as respective redundant star configurations, to permitline units and switch fabric units to be configured in “active/standby”pairs, thus supporting greater system availability and robustness.

[0024] A line unit is also disclosed which may be disposed within theline unit slot. The disclosed line unit includes an STM switch fabric,as well as an optical interface to a SONET ring. The line unit modulefurther includes two or more service unit interfaces for coupling theSTM switch fabric to point to point interfaces within the first starinterconnection configuration, so as to permit communication ofinformation contained within the SONET frames between the line unit andservice units disposed in the service unit slots. The disclosed lineunit further includes at least one ATM interface, for communicating ATMcells between the line unit and an ATM switch fabric unit disposed inthe switch fabric slot. The disclosed line unit provides what arereferred to herein as the “service affecting” functions of the devicewith regard to STM. STM service affecting functions are those functionsnecessary to maintain continued operation of STM communication throughthe device. Accordingly, to provide fault recovery and avoid STM serviceinterruptions, the device may be advantageously configured with anactive/standby pair of line units.

[0025] An ATM switch fabric unit is disclosed which may be installedwithin the switch fabric slot. The disclosed ATM switch fabric unitincludes two or more service unit interfaces which are coupled to pointto point connections within the second star interconnectionconfiguration. During operation of the device, ATM cells arecommunicated in ATM cell stream format between service units in theservice unit slots and the switch fabric unit over the second starinterconnection configuration. The disclosed ATM switch fabric unitprovides what are referred to herein as the “service affecting”functions of the device with regard to ATM. ATM service affectingfunctions are those functions necessary to maintain continued operationof ATM communication through the device. Accordingly, to provide faultrecovery and avoid ATM service interruptions, the device may beadvantageously configured with an active/standby pair of ATM switchfabric units.

[0026] A management and control unit (MCU) is disclosed which may beinstalled in the control unit slot. The disclosed MCU communicates SONEToverhead information over the third star interconnection configuration.The MCU further operates to download executable software images toservice units installed in the network element over the third starinterconnection configuration. The MCU provides what are referred toherein as the “non-service affecting” functions of the device.

[0027] A service unit for a network element is also disclosed, whichincludes a first backplane interface for connecting with the first starinterconnection configuration within the network element. The firstbackplane interface to the first star interconnection configurationpermits transport of STM frames to an STM switch fabric. The serviceunits include a second backplane interface for connecting to the secondstar interconnection configuration. The second backplane interface tothe second star interconnection configuration permits transport of ATMcells to the ATM switch fabric. In a preferred embodiment, the serviceunit further includes a third backplane interface to connect with thethird star interconnect configuration for communication with the MCUwithin the network element.

[0028] Thus there is provided a communication device which combines thefunctions of a SONET add-drop multiplexer with the functions of an ATMswitch. The disclosed device supports multiple configurations, includingSTM only, ATM only, or hybrid STM/ATM operation. Moreover, the discloseddevice is flexible and scalable such that functionality may be added ormodified as the needs of the customer change over time. The disclosedsystem advantageously applies low voltage, complementary signalingtechniques such as Low Voltage Differential Signaling (LVDS) to providehigh speed, serial point to point links in star configurations. The useof serial point to point links supports failure isolation, since failureof a single non-hub unit will not affect the connections of other unitsto the hub of the star. Accordingly, replacement of a non-hub unit ispossible without disturbing the operation of the other units in thestar. The disclosed system supports failure protection in hub units,such as the line units and ATM switch fabric units, by providingconnectivity for active/standby unit pairs of the line unit and ATMswitch fabric unit. In addition, by use of multi-function service unitslots, which can also serve as ATM switch fabric unit slots, thedisclosed system supports a wide variety of configurations in a minimumamount of space.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0029] The invention will be more fully understood by reference to thefollowing detailed description of the invention in conjunction with thedrawings, of which:

[0030]FIG. 1 shows module partitioning in an illustrative embodiment ofthe disclosed network element;

[0031]FIG. 2 shows the layout of slots in an illustrative embodiment ofthe disclosed network element interconnection system;

[0032]FIG. 3 illustrates multiple star interconnection configurationsemployed in the disclosed network element;

[0033]FIG. 4 shows an illustrative configuration of the disclosednetwork element providing synchronous transfer mode (STM) support;

[0034]FIG. 5 shows an illustrative configuration of the disclosednetwork element providing asynchronous transfer mode (ATM) support;

[0035]FIG. 6 shows an illustrative configuration of the disclosednetwork element providing asynchronous transfer mode (ATM) support andsynchronous transfer mode (STM) support;

[0036]FIG. 7 is a functional block diagram of an illustrative line unitemployed in the presently disclosed network element;

[0037]FIG. 8 is a block diagram illustrating the STM and ATM inbounddatapath within the line unit;

[0038]FIG. 9 is a block diagram illustrating the STM and ATM outbounddatapath within the line unit;

[0039]FIG. 10 is a block diagram of a control architecture for the lineunit;

[0040]FIG. 11 is a block diagram of a signal routing ASIC;

[0041]FIG. 12 is a block diagram of an extended synchronization module;

[0042]FIG. 13 is a block diagram of software and hardware used todownload a software image into a management and control unit;

[0043]FIG. 14 is a block diagram showing hardware components within amanagement and control unit and an intelligent service unit;

[0044]FIG. 15 is a block diagram showing interfaces of a management andcontrol unit;

[0045]FIG. 16 is a block diagram showing a gateway network element and aremote network element; and

[0046]FIG. 17 is a flow chart showing steps performed to download asoftware image to a network element.

DETAILED DESCRIPTION OF THE INVENTION

[0047] A network element (NE) architecture is disclosed whichconveniently and efficiently combines the functions of a SONET add-dropmultiplexer (ADM) with the functions of an ATM switch. As shown in FIG.1, the disclosed network element 10 may be physically embodied as a setof hardware “units” interconnected across an interconnection system,also referred to as a backplane. The units, for example, include lineunits (LU) 30, a management and control unit (MCU) 32, ATM switch fabricunits (ATMU) 34, and service units (SU) 36. The ATMU 34 provides ATMcellrelay related functionality, such as: VP and VC switching,segmentation and reassembly, signaling, routing, call control, trafficmanagement, and Operations Administration, Management, and Provisioning(OAMP). In this regard, the ATMU may specifically provide addresstranslation, an application programming interface (API) for user tonetwork interface (UNI) signaling, an interim local management interface(ILMI) server, full ILMI and user network interface/network to networkinterface (UNI/NNI) signaling stacks, a private node-to-node interface(PNNI) server for routing, and connection admission. In an alternativeembodiment, such ATM functionality may be split across two separate unittypes: ATM Switch units (ATMSU) containing primarily the ATM switchfabric, and ATM Processing units (ATMPRU) providing other ATM functions.

[0048] In another alternative embodiment, an internet protocol (IP)switch fabric unit may be substituted for one of the ATM switch fabricunits. The IP switch fabric unit performs switching (also known as“routing”) at the IP layer of the TCP/IP protocol stack, using what isreferred to herein as an IP switch fabric. The IP switch fabric unit mayalso provide an ATM switch fabric, together with such ATM functionalityas described above.

[0049] The network element 10 provides (1) a SONET line interface on aline side 15, (2) connection of ATM traffic from either the line side 15or a service side 20 to an ATM switch fabric unit 34, (3) connection ofSTM traffic from either the line side 15 or the service side 20 toSTS/VT switch fabrics within the line units 30, (4) various serviceinterfaces on the service side 30 through the specific service units 36.The ATM switch fabric unit 34 performs activities associated with ATMcell-relay. These for example include VP and VC switching, signaling,routing, call control, and traffic management.

[0050] The line units 30 may support various SONET optical media lineinterfaces, such as OC-3, OC-12, or any other suitable opticalinterface. The line unit provides interfaces to the ATM switch fabricunit 34 from a SONET ring on the line side 15, as well as to the variousservice units 36. The line unit also includes an STM switch fabric 39capable of performing STM switching at both the STS and VT levels.Examples of ATM switch fabric unit 34 include modules providingconnectivity to the line units 30, as well as to the service units 36,and which also include an ATM switch fabric 41.

[0051] Service units 36 include modules supporting varioustelecommunication or data communication interfaces to the service side20, including for example DS-1, DS-3, Token Ring, FDDI (fiberdistributed data interface), 10BaseT, 100BaseT, 10BaseF, or 100BaseFEthernet, EC-1 (electrical carrier 1, also referred to as STS-1electrical, or STS-1/EC-1), OC-3, OC-12 or any other suitable service.The service units 36 may communicate with the ATM switch fabric 41installed in the ATM switch fabric unit 34 of the network element, aswell as with the STM switch fabrics 39 contained within the line units30. Accordingly, service units 36 for the network element 10 containinterfaces within the network element which may be considered to fallwithin three categories: STM, ATM, and STM/ATM. For example, STM serviceunit 36 a formats data received from the service side 20 into STS framesto be forwarded to the line units 30, while ATM service unit 36 cformats data received from the service side 20 into ATM cell streams tobe forwarded to the ATM switch fabric unit 34. Since each service unit36 is connected to both ATM and STM services, a single STM/ATM serviceunit 36 b may simultaneously use ATM and STM switching services byselectively communicating the data it receives to the line units 30and/or the ATM switch fabric unit 34.

[0052] Management and control unit 32 includes a subsystem employing amicroprocessor 33 coupled to microprocessor bus 37. The illustrative MCU32 provides a basic control infrastructure for the network element 10,using what is referred to herein as a “serialized hardbus”. Duringoperation of the network element 10, the components of the serializedhardbus convert a parallel bus communication protocol into amaster/slave, serialized communication between the MCU 32 and the otherunits in the network element 10. A serialized hardbus master logic unit35 is coupled to microprocessor bus 37 of the MCU and maps control andstatus registers and/or memory locations of other units in the networkelement onto the microprocessor memory map. Units within the device 10which are managed by the MCU 32 include serialized hardbus terminationlogic. The serialized hardbus also provides notification of certainautonomous events and/or alarms, occurring on or detected by otherunits, by interrupting the microprocessor 33.

[0053] In addition to the serialized hardbus, the MCU 32 uses a SONEToverhead link from each line unit 30 and service unit 36, to supportmaintenance communications such as Data Communications Channel (DCC) andOrderWire information. The MCU 32 further includes various managementand control capabilities, such as DCC and Orderwire processingfunctionality, which may include a combination of hardware and software,and which are used to process the received maintenance information.

[0054]FIG. 2 shows an illustrative slot layout 40 for the disclosedinterconnection system. The slot layout 40 of FIG. 2 includes severaldifferent types of slots, specifically line unit slots 42, managementand control unit slot 44, and service unit slots 46. The service unitslots 46 include two slots 48 which may also be used as ATM switchfabric slots. The slots 48 are accordingly also referred to as switchfabric slots, or ATM switch fabric slots. The slots shown in FIG. 2 areeach operable to receive hardware units as described in FIG. 1 of thecorresponding type. For example, line unit slots 42 are operable toreceive line units 30, service unit slots 46 are operable to receiveservice units 36, ATM switch fabric unit slots 48 are operable toreceive ATM switch fabric units 34 or service units 36, and MCU slot 44is operable to receive the MCU 32.

[0055]FIG. 3 illustrates the multi-star architecture of the disclosedinterconnection system. As shown in FIG. 3, a number of point-to-pointconnections 50 form star configurations having the management & controlunit slot 44, line unit slots 42, and ATM switch unit slots 48 as hubs.In a preferred embodiment, the point-to-point connections 50 areimplemented using a low voltage, complementary signaling technique toachieve high speeds, such as Low Voltage Differential Signaling (LVDS).In addition, the point-to-point connections 50 are terminated in thebackplane, without the presence of a service unit within any of therespective service unit slots 46. Accordingly, no additional physicaltermination is required for empty one of the service unit slots 46.

[0056] In the depiction of FIG. 3, and as shown in FIG. 2, two of theservice unit slots 46 are the same as ATM switch fabric slots 48.Accordingly, when ATM switch fabric slots 48 are used to connect ATMswitch fabric units, these same slots may not simultaneously be used toinstall service units. However, as shown in FIGS. 2 and 3, those two“multifunction” slots may alternatively be used to connect service unitsto the device in configurations where the ATM switch fabric units arenot needed.

[0057] More specifically, in a preferred embodiment, each line unit slot42 has two star configurations emanating from it. An STM datapath starcommunicates STM type data with each of the service unit slots 46. Thesecond star emanating from each line unit slot 42 is a synchronizationstar for conveying a frequency reference and a frame alignment pulse(FP) to the service unit slots 46, in order to synchronize STMcommunication between each service unit and the STM switch fabricswithin the line units. An extended synchronization module (ESM) may beprovided to accept and distribute an extracted clock from the serviceunit slots 46 or, alternatively, a BITS clock from furtherinterconnections within the device. The ESM is for example implementedon a sub-board module which is electrically and mechanically coupled toan alternative, enhanced version of the illustrative line unit.

[0058] In addition to the STM datapath and synchronizationinterconnection stars, a private datapath 52 is provided forcommunicating STM data between active and standby line units installedin respective ones of the line unit slots 42. The private datapath 52facilitates a pass through path between the line units which may berequired under certain conditions.

[0059] Further, in the illustrative embodiment of FIG. 3, the line unitslots 42 each have a datapath connection 54 to each of the ATM switchfabric slots 48. This interface supports ATM cell stream traffic betweenthe ATM switch fabric slots 48 and the line unit slots 42. Duringoperation of the device, ATM traffic from both line unit slots 42(active and standby) is forwarded to each of the ATM switch fabric slots48. Accordingly, per unit fault protection of the units in the line unitslots 42 and ATM switch fabric slots 48 may be provided independently,on a unit by unit basis.

[0060] Further, in the illustrative embodiment of FIG. 3, the MCU slot44 has two star interconnect configurations emanating from it. First, anMCU control star is provided to support the serialized hardbus betweenthe MCU and each other unit in the device. Second, a SONET overhead starsupports communication of SONET overhead information, such as DCC andOrderwire data, as well as other information in the section, line, orpath overhead portions of a SONET signal, between the MCU and otherunits in the device. The SONET overhead star connects the MCU slot 44 toeach other slot in the device, via respective ones of the point to pointconnections 50, thus enabling communication of SONET overhead betweenthe MCU and any other unit in the device, including service unitsinstalled in the service unit slots 46. The MCU slot 44 is furtherprovided with a software image download interconnection with the ATMswitch fabric slots 48 to facilitate communication of softwareexecutable image data between an ATM switch fabric unit installed in theATM switch fabric slots 48 and the MCU, for example using the High-LevelData Link Control (HDLC) protocol. Such software executable image datamay further be communicated from the MCU to individual ones of theservices units installed in the service unit slots, over the serializedhardbus.

[0061] The ATM switch fabric slots 48 each have a datapath staremanating from them to all other service unit slots 46. Because theseATM datapath stars are routed to the other service unit slots 46independently from any other star configuration, the cost of adding ATMfeatures is independent of the costs associated with other functionalityprovided by the device.

[0062] The disclosed interconnection system supports ATM VP PathSwitching within an active ATM switch fabric unit coupled to one of theATM switch fabric slots 48, through the datapath connection 54. Thedatapath connection 54 between each of the ATM switch fabric slots 48and both line unit slots 42 enables maintenance of a current switchstate in both of the ATM switch fabrics within ATM switch fabric unitsin the switch fabric slots 48. Unlike support provided in the device forthe SONET Unidirectional Path Switched Ring (UPSR), in which the passthrough path is independently carried between the line units over theprivate datapath 52, the VP pass through path is internal to an activeATM switch fabric unit. Accordingly, each ATM switch fabric slot 48supports full ATM bandwidth to and from each line unit slot 42simultaneously.

[0063] Additionally, the ATM switch fabric slots 48 connect with an ATMControl Bus which extends to two of the service unit slots 46 which areadjacent to the ATM switch fabric slots 48, such as the service unitslots labeled SUO9 and SU010 in FIG. 2. This bus enables division of theATM switch fabric unit functionality into separate ATM Switching Unitand ATM Processing Unit modules. Such a division permits the number ofmodules and power allocation for the ATM function to be doubled byoccupying 4 slots with ATM switch fabric units instead of 2.

[0064] For protected services, service unit slots 46 are allocated inpairs, to support operation of redundant service units. However, eachone of the service unit slots 46 has independent connections to both theATM switch fabric units (in the ATM switch fabric slots 48), and to boththe STM switch fabrics in the line units (in the line unit slots 42).This allows for a configuration of up to 12 different unprotectedservice units to be supported simultaneously in an STM configuration,and 10 different unprotected service units in an STM/STM configuration.Configurable backplane private connections 51 between selected pairs ofservice unit slots 46 are also provided, to support active/standbyservice unit pairs, as well as active standby pairs of ATM switch fabricunits in ATM switch fabric slots 48.

[0065]FIGS. 4 through 6 illustrate various system configurations thatmay be obtained using the presently disclosed architecture. Thedisclosed network element can be configured as an STM system as shown inFIG. 4, an ATM system as in FIG. 5, or an STM/ATM hybrid system as shownin FIG. 6, depending on the complement of units which are employed.

[0066] The exemplary configurations of FIGS. 4-6 include the followinghardware components:

[0067] 1) line units (LUs) 60 a and 60 b, which operate as anactive/standby pair, each of which terminates a SONET ring, for examplethrough an OC-12 connection.

[0068] 2) a number of service units (SUs), illustrated by the followingspecific examples:

[0069] STM Service Units 64, which map STM services provided by thedevice to their respective STM service interfaces. Similarly, ATMsubtending ring service units 80 e and 80 f provide a service sideinterface to a SONET ring carrying ATM cells, and STM/ATM subtendingring service units 90 a and 90 a provide a service side interface to aSONET ring carrying both STM and ATM traffic.

[0070] Service units 90 a and 90 a may also be referred to as “HybridService Units”, because they provide both STM and ATM services to theirrespective service interfaces. A hybrid service unit includes threeinterfaces: an STM internal interface (to the STM switch fabric), an ATMinternal interface (to the ATM switch fabric), and a service interface.During operation of a hybrid service unit, data units such as packets orcells are forwarded from the interface at which they are received toeither of the other interfaces. Such forwarding is performed by one ormore application specific integrated circuits (ASICs) and/or amicroprocessor based subsystem. For example, in another illustrativeembodiment, an Ethernet hybrid service unit is provided. In the Ethernethybrid service unit, data units carried in Ethernet frames are receivedat an Ethernet service interface, and are selectively forwarded toeither the STM or the ATM interfaces, based on information such asaddressing included within the header of each received Ethernet frame.Similarly, each data unit received at the ATM or STM internal interfacesmay be forwarded to either the other internal interface, or to theEthernet service interface, responsive to information contained withineach data unit, or to provisioning of individual signals. Hybrid serviceunits may include any suitable external service interfaces to thedevice, including but not limited to data communications network, suchas Ethernet or ATM, or any traditional telecommunications system, suchas the digital multiplex hierarchy (DMH) or SONET.

[0071] ATM Interworking Service Units 80 a, 80 c and 80 d, which adapttraditional datacom (10/100BaseT for example) or telecom (OC-3 or DS-1for example) service interfaces to the ATM protocol.

[0072] Native ATM Service Unit 80 b, which provides an interface to theATM services provided by the device to an ATM service interface, forexample an OC-3 based cell relay connection.

[0073] 3) ATM switch fabric units (ATMUs) 70 a and 70 b, which form anactive/standby pair, and which handle ATM VP/VC switching and otheractivities associated with ATM cell relay, such as signaling, routing,call control, and traffic management.

[0074] 4) A Management and Control Unit (MCU) 62, which manages andcontrols the units within the network element 10. This unit provides allthe administrative interfaces to the device and processes all the SONEToverhead bytes.

[0075] The STM system of FIG. 4 includes the two LUs 60 a and 60 b, theMCU 62, and some number of STM service units 64 a-64 e. Service units 64a and 64 b are an active/standby pair supporting a SONET DS-1 serviceinterface, service units 64 c and 64 d are an active/standby pairsupporting a SONET DS-3 service interface, and service unit 64 esupports a SONET OC-3 service interface. The line units 60 a and 60 battach via OC-12 to a SONET ring 61. During operation of the embodimentshown in FIG. 4, STM signals are routed by the STM switch fabric in theactive one of the line units 60, between the ring 61 and active ones ofthe service units 64 over individual ones of point to point STS-3 seriallinks 66.

[0076]FIG. 5 shows an ATM configuration of the disclosed networkelement, including the two LUs 60 a and 60 b, the ATMUs 70 a and 70 b,the MCU 62 and ATM service units 80 a through 80 f. The ATM Interworkingservice unit 80 a includes a service interface to a 10/100BaseT LAN, thenative ATM service unit 80 b includes a service interface to an OC-3based cell relay connection, and a pair of ATM Interworking ServiceUnits 80 c and 80d include service interfaces to a DS-1 frame relayconnection. The service units in FIG. 5 are connected via anactive/standby pair of ATMUs 70 a and 70 b to a pair of OC-12 LUs 60 aand 60 b. The service units 80 c and 80 d are configured as anactive/standby pair. The ATM subtending ring service units 80 e and 80 fprovide a service side interface to a SONET ring carrying ATM cells.

[0077] During operation of the embodiment shown in FIG. 5, ATM cellscarried over STS signals within the SONET ring 61 are routed by the lineunits 60 a-60 b over STS-12 datapath connections 81 to each of the ATMswitch fabrics within the ATM switch fabric units 70 a and 70 b. The ATMswitch fabric units 70 a and 70 b in turn direct the ATM cells, based onVPI/VCI values within the cell headers, to the appropriate destinationservice units as indicated by ATM virtual connections establishedthrough the ATM switch fabric.

[0078] A hybrid STM/ATM configuration of the disclosed network elementis shown in FIG. 6. In FIG. 6, the STS signals from the ring 61 whichcontain encapsulated ATM cells are routed by the line units 60 a-60 b tothe ATM switch fabric units 70 a-70 b. STS signals from the ring 61 thatare provisioned to pass through service interfaces of the device arerouted by the line units 60 a-60 b to the appropriate service unitsusing the STM switch fabrics contained within the line units 60 a-60 b.The service units 64 a-64 b communicate STM frames with the line units60 a-60 b, while the service units 80 a-80 b communicate ATM cells withthe ATM switch fabric units 70 a-70 b. The service units 90 a-90 acommunicate ATM cells with the ATM switch fabric units 80 a-80 b, andalso communicate STM frames with the line units 60 a-60 b.

[0079]FIG. 7 is a functional block diagram of an illustrative line unit100. The line unit 100 provides an optical interface to a SONET ring atthe line side of the disclosed network element. The line unit 100 isshown including a signal routing ASIC 102, which is coupled to anoptical receiver 104, an optical transmitter 106, ATMU Low VoltageDifferential Signaling (LVDS) receivers 108, ATMU transmitters 110, andan STM switch fabric ASIC 114. ATMU transmitters 110 and ATMU receivers108 are, for example, shown using LVDS devices. The STM switch fabricASIC 114 is shown including service unit transmit interfaces 116 andservice unit receive interfaces 118.

[0080] During operation of the line unit 100, the optical receiver 104receives for example a SONET formatted OC-12 or OC-3 optical signal,carrying an STS-12 or STS-3 signal respectively, or any suitablyformatted signal. The optical receiver 104 passes electrical clocksignals 124 and data signals 126 that reflect the received SONET signalto the signal routing ASIC 102. The signal routing ASIC 102 extracts STSframes from the STS signal, performs pointer interpretation to locatethe beginnings of payloads and virtual tributaries within the receivedframes, and also extracts line, section and path overhead data. Theextracted overhead data 128 is sent by the signal routing ASIC 102 to amanagement and control unit (MCU) for further processing.

[0081] According to provisioning information provided to the line unit100 by the MCU, the signals in the received STS signal are interpretedby the line unit 100 as carrying either ATM or STM traffic. The STS-1signals provisioned as ATM traffic are processed by the line unit 100 toperform the ATM Transmission Convergence (TC) layer functions of celldelineation and STS channel identification. The resulting ATM cells arethen formatted into a cell stream and sent to the active and standby ATMswitch fabric units, via transmitters 110.

[0082] The STS signals received by the line unit 100 that carry STMtraffic are sent by the signal routing ASIC 102 via connections 130 tothe STM switch fabric ASIC 114, where STS/VT grooming andcross-connection takes place. The switch fabric ASIC 114 is furthercoupled via connection 132 to receive STM traffic from a signal routingASIC of a “partner” line unit in an active/standby pair. Similarly, theoutput connection 134 of the signal routing ASIC 102 is used to pass STMtraffic to the switch fabric of the partner line unit.

[0083] The switch fabric ASIC 114 receives STM traffic from the lineside of the device through one of the two connections 130 and 132, eachof which for example provides full STS-12 bandwidth, and from theservice side through service unit receive interfaces 118, each of whichfor example provides STS-3 bandwidth. The switch fabric ASIC 114 outputsSTM signals through the output 135 (STS-12) or any of the service unittransmit interfaces (STS-3) 116.

[0084] Further during operation of the line unit 100 as shown in FIG. 7,the switch fabric ASIC 114 performs Unidirectional Path Switched Ring(UPSR) path selection, responsive to path performance informationprovided by the signal routing ASIC 102, and passed to the switch fabricASIC 114 within some number of over-written overhead bytes, in order todetermine which of the connections 130 or 132 should be the path forindividual STS or VT signals that are dropped at the service side of thedevice. The switch fabric ASIC 114 performs this selection in responseto path performance criteria, in order to determine the path of highestquality from the two available paths. The switch fabric ASIC 114performs this selection at either the STS-1. or VT 1.5 level for eachpath dropped to the service side of the device.

[0085] The line unit 100 also includes a back-up memory 141 which storesconfiguration information that may be used in the event of a failure ofthe MCU. The contents of the back-up memory 141 may also be used by anew MCU that is installed to replace the failed unit. The physicalinventory EEPROM 142 is used to store information such as the serialnumber of the unit, a hardware revision number, and a software revisionnumber. A serial bus terminator SBT 143 operates to connect the unit tothe serial hardbus for communication with the MCU, and LEDS 144 providea visual indication of the unit's status.

[0086] The line unit 100 further includes various clock related elementsincluding an extended synchronization module (ESM) 120, which, incombination, provide STM synchronization clocks to the line unit andother units within the device. Specifically, a timing reference switch145 is controlled by the output of synchronization switch controllerlogic 146. A number of inputs 147 to the synchronization switchcontroller logic 146 provide indication of whether the ESM 120 ispresent, whether the active/standby partner line unit is present, andwhether the SONET signal on the line side is present on either the localor partner line unit.

[0087] The inputs 148 to the timing reference switch 145 include a SONETminimum clock (SMC) 149 generated by a clock source on the line unit,and a local line reference clock from the signal routing ASIC 102, thatis derived from the line side SONET ring. SMC and line reference signalsare also provided to the timing reference switch 145 from the partnerline unit as well. When the ESM daughter board is present, the referencesignal from the ESM is always selected to pass through the timingreference switch 145.

[0088] The output of the timing reference switch 145 is passed todistribution phase locked loop 151 which smoothes out any switchingtransients and converts the selected timing reference to a higherdistribution frequency that is passed to the other units in the device,as well as to frame pulse generation logic 153. The frame pulsegeneration logic 153 derives a frame pulse from the distributionfrequency and passes that frame pulse to the other units in the device,as well as to the frame pulse generator of the partner line unit. Thedistribution frequency is also received by the board clock logic 155,and passed to phase locked loops 157 and 159 for further frequencyconversion to obtain frequencies needed to support the ATM and STMprotocols within the logic of the signal routing ASIC 102 and switchfabric ASIC 114.

[0089]FIG. 8 is a block diagram illustrating the operations performed inthe STM and ATM inbound datapaths within the line unit 100 of FIG. 7. Anincoming SONET signal is first converted from optical to electricalsignals (OC-12 to STS-12 for example) by optical to electricalconversion circuit 150. The output of the circuit 150 is passed to thesignal routing ASIC 102, which performs section, line and path overheadextraction, as well as STM and VT pointer processing in a firstfunctional block 152. In response to provisioning information, thefunctional block 152 separates ATM and STM traffic contained within thereceived SONET signal. A first output of the functional block 152 is ATMtraffic 154 extracted from the received SONET signal, which is passed toa second functional block 156 within the signal routing ASIC 102. Asecond output of the functional block 152 is STM traffic 158 extractedfrom the received SONET signal, which is passed to both the STM switchfabric of the partner line unit, and also to the local switch fabricASIC 114. A third output of the functional block 152 is the extractedSONET overhead information 128, which is passed to the MCU.

[0090] The signal routing ASIC 102 further provides path selectioncontrol information to the switch fabric ASIC 114, as well as to the ATMswitch fabric units. Such information reflects monitoring of incomingpath performance by the signal routing ASIC 102, for example through biterror monitoring, AIS (Alarm Indication Signal) monitoring, alarmdetection, and or path label monitoring. The path selection informationprovided to the switch fabric ASIC 114 may be at the STS signal or VTlevel. Path selection information at the STS level is provided bywriting a path status value over an STS line overhead byte within theSTS signal. The status value reflects which of the input paths to theswitch fabric ASIC 114 is to be used to receive the particular STSsignal associated with the line overhead byte. The line overhead bytemay be overwritten by the signal routing ASIC because it has previouslyextracted the line overhead and sent it to the MCU, prior to forwardingof the signal to the switch fabric ASIC 114. For VT specific pathselection information, the signal routing ASIC 102 overwrites one of theVT pointer bytes for the corresponding VT with path status information.The VT pointer byte may be overwritten by the signal routing ASIC 102because it has previously performed VT (as well as STM frame) pointerprocessing. In this way STM traffic 158 carries path selectioninformation to the STM switch fabrics of both the local and partner lineunits. The signal routing ASIC 102 must align the STM traffic 158 withthe STM traffic 132 received from the signal routing ASIC of the partnerline unit prior to reception by the switch fabric ASIC 114. Suchalignment must ensure that STS signals received by the switch fabricASIC 114 are frame aligned for STS signals, and frame and SPE alignedfor VTs.

[0091] The path selection control information provided by the signalrouting ASIC 102 to the active and standby ATM switch fabric units issimilarly reflective of incoming path performance and availabilitymonitoring as described above. The path selection control informationfor the ATM switch fabric unit is output from the signal routing ASIC102, and transmitted as part of a serial data stream including an ATMcell stream to each of the ATM switch fabric units. The path selectioninformation so provided is used by the ATM switch fabric units toperform ATM VP path selection.

[0092] The second functional block 156 in the signal routing ASIC 102formats the ATM traffic 154 into an ATM cell stream. The resulting cellstream is stored in the output buffer 160 for transmission to the activeand standby ATM switch fabric units. While the output buffer 160 isshown for example within the signal routing ASIC 102 in FIG. 8, they mayalso be implemented externally to the signal routing ASIC 102.

[0093] The STM traffic 158 from the signal routing ASIC 102 is receivedby the switch fabric ASIC 114, where the STS-1 signals it contains aregroomed for the type of traffic they are carrying. For example, a VTmapped STS-1 signal in the STM traffic from the signal routing ASIC isbroken up into 28 VTs 162, which are organized into two sets of 14 VTs.Each set of 14 VTs is received from both the local STS switch fabric 163and the partner STM switch fabric by the path selector logic 166 forsubsequent broadcast to an active/standby service unit pair. Inaddition, an STS-1 signal 164 in the STM traffic from the local andpartner signal routing ASICs is shown for purposes of example beingpassed to the path selector logic 166, also for subsequent broadcast toan active/standby service unit pair.

[0094] The path selector logic 166 selects the best receive path betweenthe path from the local STM switch fabric, and the path 132 from the STMswitch fabric in the partner line unit, in response to in-band pathselection information provided by the signal routing ASICs 102 of boththe active and standby line units. The path selection information iscontained within overwritten overhead bytes in each particular STS or VTsignal. The outputs of path selector logic 166 are then combined bymultiplexers 167 for STS-3 transmission to the appropriate serviceunits.

[0095]FIG. 9 is a functional block diagram illustrating operationsperformed in the STM and ATM outbound datapaths within the illustrativeline unit 100 of FIG. 7. FIG. 9 shows outbound datapath operation for aline unit provisioned to insert 1 VT-mapped STS-1 signal 190, one DS-3mapped STS-1 signal 192 and one ATM mapped STS-1 signal 194 onto anoutgoing STS-12 signal 196, which is converted to OC-12 by electrical tooptical conversion circuit 175. The 28 VTs of the VT-mapped STS-1 signal190 are received from two separate active/standby pairs of DS-1 serviceunits 184 and 186. The DS-3 source of the DS-3 mapped STS-1 is anactive/standby pair of DS-3 service units. The ATM mapped STS-1 signal194 is received by the line unit 100 via cell streams 181 from theactive/standby ATM switch fabric units within the device.

[0096] The 28 VTs from service units 184 and 186 pass through STS switchfabric 163 to path selector logic 166 where selection between active orstandby service unit sources is performed in response to path selectioninformation contained within the respective VT and STS signals receivedfrom the service units. In an illustrative embodiment, each one of theservice units includes a signal routing ASIC such as signal routing ASIC102, which embeds path selection information within each VT or STSsignal in response to path performance and availability monitoring.

[0097] The 28 VTs are then multiplexed onto a single STS-1 signal 190 bymultiplexers 169, and passed to the signal routing ASIC 102. The DS-3mapped STS-1 also goes through the STS switch fabric 163 to pathselectors within path selector logic 166, also for selection betweenactive or standby service unit sources. The DS-3 mapped STS-1 is thenpassed as STS-1 signal 192 to the signal routing ASIC 102.

[0098] Receive buffers 178 within the signal routing ASIC 102 receivethe two ATM cell streams 181 from the active and standby ATM switchfabric units (ATMU-A and ATMU-S). While the receive buffers 178 areshown within the signal routing ASIC 102 in FIG. 9, they may also beprovided externally to the signal routing ASIC 102. A parity bit isincluded with each cell stream, and is used by the signal routing ASIC102 to monitor the quality of the paths carrying the cell streams to thesignal routing ASIC 102. One of the cell streams 181 is selected,responsive to path quality information, to pass from receive buffering178 to a first logic block 180. Within the logic block 180, ATMTransmission Convergence layer functions are performed, the cell streamis delineated, and the STS routing tags are stripped off. The resultingATM-mapped STS-1 signal 194 is then processed by functional block 182,and multiplexed onto the line side SONET ring with the other STS-1's 190and 192. Section, line, and path overhead information are inserted asnecessary, and the combined traffic is sent to the electrical to opticalconversion circuit 175 for optical transmission onto the SONET ring. Forexample, STM traffic 190 and 192 includes path overhead informationcarried through the STM switch fabric, and requires line and sectionoverhead to be inserted by the signal routing ASIC 102. ATM traffic 194requires path, line and signal overhead bytes to be inserted by thesignal routing ASIC 102.

[0099] An illustrative control architecture for a line unit is shown inFIG. 10. FIG. 10 shows a serial hardbus terminator 200 coupled to anaddress/data bus 202. A number of general purpose I/O registers 208 arefurther coupled to the address/data bus 202, and made accessible to theMCU over the serial hardbus via the serial hardbus terminator logic 200.A memory map 206 consisting of accessible registers in the switch fabricASIC 114, and a memory map 204 consisting of accessible registers in thesignal routing ASIC 102 are also made accessible to the MCU via theserial hardbus terminator logic 200. During operation of the line unit,the MCU provides provisioning information through the serial hardbus tothe line unit. The provisioning information determines the operation ofthe signal routing ASIC 102 and switch fabric ASIC 114. For example,provisioning information from the MCU controls how the switch fabricASIC 114 routes STS signals it receives, and determines which of the STSsignals on the line side SONET ring are treated as containing ATM cells.In addition, the MCU detects failures and error conditions throughinterrupts received over the serial hardbus. In response to detection ofsuch failures and error conditions, the MCU provides path selectioninformation to the line unit over the serial hardbus, for exampledetermining which one of an active/standby pair of service units is tobe the source for a particular VT at any given time.

[0100]FIG. 11 shows an illustrative embodiment of the signal routingASIC 102 provided within the line unit, and which may also be providedwithin a service unit. The signal routing ASIC 102 is capable ofreceiving data from and transmitting data to optical drivers thatinterface to a SONET ring on the line side or the service side of thedevice. The signal routing ASIC 102 further contains an ATM interfacefor processing ATM cells, such as are communicated between a line unitand an ATM switch fabric unit. Finally, the signal routing ASIC 102includes an STM interface for transmitting data to and receiving datafrom an STM switch fabric ASIC.

[0101] The signal path for data received by the signal routing ASIC 102from the SONET ring is first described. Data received from the SONETring is converted from an optical signal to an electrical signal by anoptical receiver outboard of the signal routing ASIC 102, and isserially coupled to one input of a line selector 225 within the signalrouting ASIC 102. In a preferred embodiment, the serial input to theline selector 225 comprises a 155 megabit per second unidirectionalpath. The signal at the serial input to the line selector 225 may forexample be 1 STS-3c signal, 3 STS-1 signals, 84 VTs, or any combinationof STS-1 and VT signals within the specified signaling bandwidth.

[0102] The signals received from the SONET ring are passed from theoutput of the line selector 225 to both a synchronous payload envelope(SPE) splitter 227, and an overhead bit drop (OHB) recovery circuit 229.Overhead bits carried on the received STM signal are segregated by theOHB recovery circuit 229 and coupled to a RX OHB Serial Link interface231. In a preferred embodiment, the RX OHB Serial Link interface 231transmits the overhead bits of the signals on the SONET ring over a 9.72mbps serial link to the MCU for processing.

[0103] The SPE splitter 227, in response to previously receivedprovisioning information from the MCU, extracts STM, VT, and/or ATMtraffic streams from the STS signals on the SONET ring, and passes eachone of these traffic streams into a respective one of three elasticstores. In the case of ATM traffic, the ATM cells are unencapsulatedprior to being stored in the ATM cell FIFO 233.

[0104] More specifically, the output of the synchronized payloadenvelope splitter 227 is coupled to three elastic stores. The elasticstores are fabricated as first in first out storage (FIFOs) and serve arate decoupling function. In the preferred embodiment, the first elasticstore 235 receives STS-1 and STS-3c traffic and is used to perform ratedecoupling for such traffic. The second elastic store 237 receives VTtraffic. The third elastic store is an ATM cell FIFO 233, and isemployed to provide rate decoupling for ATM cell traffic. Outputs of theof the first and second elastic stores 235 and 237 are coupled to asynchronized payload envelope multiplexer (SPE MUX) 239, which isemployed to multiplex the various STS signals to the STM switch fabricson each of the active/standby line unit pair. The output of the SPE MUX239 is broadcast to the STM switch fabrics on both active and standbyline units in the event two such cards are provided for purposes ofredundancy.

[0105] The output of the ATM cell FIFO 233 is coupled to the ATM receivelink interface (ATM Rx Link) 241. The output of the ATM receive linkinterface 241 is coupled to the ATM switch fabric unit via a four bitwide parallel data bus which is clocked at 39 Mhz to a 156 Mb/sbandwidth between the signal routing ASIC and the ATM switch fabric.Since, in the illustrative embodiment of FIG. 11, the bandwidth betweenthe signal routing ASIC 102 and the ATM switch fabric unit exceeds theingress bandwidth of 155 Mb/s, the ATM cell FIFO 233 cannot overflow. Inthe foregoing manner, ingress SONET traffic carrying STS-1, STS-3c, VTor encapsulated ATM cells is forwarded to either an STM or ATM switchfabric as applicable.

[0106] Data destined for the line side SONET interface is received bythe signal routing ASIC 102 from either the STM switch fabric or the ATMswitch fabric. More specifically, an active/standby switch 243 receivesSTM data from both a first STM switch fabric, for example which isco-resident on the local line card, as well as from a second STM switchfabric, for example located on a separate line card, in the event theseparate line card is present and configured with the first line card asa redundant pair. The active/standby switch 243 is controlled byprovisioning information received from the MCU to select STM data fromone of two STM switch fabrics to which is it coupled. The selected STMdata is passed to an SPE multiplexer 245.

[0107] Similarly, ATM cells destined for a SONET interface of the deviceare received at an ATM Transmit Link interface 247 either from a singleATM switch fabric or from two ATM switch fabrics (active and standby) inthe event that two ATM switch fabrics are provided in a redundantconfiguration. In a preferred embodiment, ATM data is received from therespective active/standby ATM switch fabrics over a 4 bit wide parallelinterface running at approximately 39 mbps. The ATM data received fromone of the active/standby pair of ATM switch fabric units is selected,and passed to the ATM cell FIFO, which performs rate decoupling. The ATMdata is then passed on to the SPE MUX 245. The SPE MUX 245 multiplexesthe ATM cell data from the ATM cell FIFO 249 together with the STM datafrom the active/standby switch 243 for subsequent STM transmission.

[0108] The output of the SPE MUX is then coupled to an overhead bytemultiplexer (OHB MUX) 251. The OHB MUX 251 also receives as an inputmanagement and control information, generated by the MCU, via a transmitOHB serial link interface 253 to the MCU. The OHB MUX 251 inserts thereceived overhead information in the appropriate overhead channels ofthe resulting SONET signal.

[0109] In a preferred embodiment, the output of the OHB MUX 251 isprovided both in the form of an 8 bit parallel interface, as well as toa parallel to serial converter 257, which provides a serial outputinterface 259. The parallel interface to the signal routing ASIC 102 maybe coupled to a parallel bus, for example in an alternative embodimentin which the signal routing ASIC is included within a service unit. Theparallel data would then be processed as required by the particularservice unit. The serial output 259 is illustratively coupled to anelectrical to optical converter, as in the preferred embodiment in whichthe signal routing ASIC is employed within the line units of the device.The optical converter then passes the serial data to the line side SONETinterface. In the foregoing manner, the signal routing ASIC 102 iscapable of routing STM and ATM formatted traffic between a line sideSONET interface, ATM switch unit interface, and STM switch fabricinterface.

[0110] In an alternative embodiment of the signal routing ASIC 102 shownin FIG. 11, which is designed to support a line side interface to anOC-12 SONET signal, the second elastic store 237 is provided between thefirst elastic store 235 and the SPE multiplexer 239. Such aconfiguration allows the second elastic store 237 to utilize the sameclock domain as the output of the first elastic store 235 and the inputof the SPE multiplexer 239, in order to eliminate the clock jittereffect and reduce the number of gates needed to fabricate the signalrouting ASIC 102.

[0111]FIG. 12 shows an extended synchronization module (ESM), includinga timing source selection ASIC 312. The ASIC 312 is shown having inputsincluding a 12.96 Mhz clock source 300 from an ESM on a partner lineunit, as well as 16 inputs 302 from each service unit in the device. Apair of 1.544 Mhz clocks, which are output from a T1 receiver 308, arealso input to the ASIC 312. The T1 receiver 308 is operable to receivean externally generated 1.544 Mhz BITS clock 310.

[0112] The ASIC 312 selects one of the input clocks it receives at eachof multiplexers 311, and detects which of the input clocks are notpresent in loss of signal (LOS) circuit 313, and informs the digitalphase locked loop 320 of any such loss. The selected clock is passed tofrequency measuring logic 314, fractional divider 316 and fractionaldivider 318 within the ASIC 312. Frequency measuring logic 314 comparesthe selected clock with a target clock based on its own internal timebase. The results of this comparison are passed to the digital phaselocked loop 320. The ASIC 312 may select a new clock source usingmultiplexers 311, in response to feedback control signals from thedigital phase locked loop 320.

[0113] The fractional divider circuit 316 derives an 8 Khz referenceclock from the selected clock source, which is passed to the digitalphase locked loop 320. The fractional divider 318 derives a 1.544 Mhzclock from the selected clock source, which is passed to the T1 receiver308, in order to provide an alternative 1.544 Mhz source to the T1receiver 308.

[0114] The digital phase locked loop 320 generates a 3.24 Mhz clock,that is passed to phase locked loop 322 which converts the signal to12.96 Mhz. The converted signal is passed to protection switch logic 324and to any ESM on the partner line unit. The protection switching logic324 selects between either the 12.96 Mhz clock from phase locked loop322 or the 12.96 Mhz clock from the ESM of the partner line unit. Theselected 12.96 Mhz signal is passed through phase locked loop 326 forsmoothing and into clock distribution buffer 328 for distribution to theother units in the device.

[0115]FIG. 13 shows components for performing a software image downloadto or from an MCU 400. A craft personal computer (PC) 420, operationssupport station (OSS) 422, and remote network element (NE) 418 are showncommunicably coupled with the MCU 400 via a first communications network416. A transmission control protocol/internet protocol (TCP/IP)functional unit 410 operates in conjunction with a file transferprotocol (FTP) functional unit 408 to support file transfers between theRAM disk 404, and the OSS 422, craft PC 420, or network element 418.

[0116] An Open Systems Interconnect (OSI) protocol stack 414 operates inconjunction with a file transfer access method (FTAM) functional unit412 to support file transfer between the RAM disk 404 and the OSS 428network element 426 on a second communications network 424. A downloadagent 430 operates to maintain a software download managementinformation base (MIB) 432, as well as to process software downloadcommands 427 received by the MCU. A software management agent 434maintains version control over software images stored in either the RAMdisk 404 or the FLASH disk 406. Software image files received by the MCU400 are copied from the RAM disk 404 to the FLASH disk 406, and thenloaded into appropriate ones of service units 402, over serial hardbusconnections 403. The service units 402 each receive a respectivesoftware image initially into a “standby” RAM 440, and subsequently copyit to an “active” FLASH RAM 442. The image is then copied into storagereferred to as the “executable” RAM 444, from which the image may beexecuted on a microprocessor 446. As the active FLASH RAM 442 and FLASHdisk 406 are non-volatile stores, the software images stored within thempersist following removal of power from the device.

[0117]FIG. 14 shows hardware components within a DCC processor 500 of anMCU card, and within an intelligent service unit 502. The DCC processor500 includes a FLASH disk 504, CPU 506, “boot” FLASH RAM 508, RAM disk510, “executable” RAM 512, “active” FLASH RAM 514, “standby” RAM 516,and serial hardbus master logic unit 518. During operation, a softwareimage received by the DCC processor 500 is initially stored in RAM disk510. The image is then copied to FLASH disk 504. The FLASH disk 504maintains a software image for the MCU, as well as for each intelligentservice unit in the device, and for any ATM processor associated with anATM switch fabric unit installed in the device. A received softwareimage for the MCU itself is copied from the FLASH disk 504 to thestandby RAM 516 and then to the active FLASH RAM 514. The MCU softwareimage is subsequently passed to the executable RAM 512, from which it isexecuted on the CPU 506.

[0118] A software image for the intelligent service unit 502 is copiedfrom the FLASH disk 504 over the serial hardbus 519 to the standby RAM524 of the intelligent service unit 502, and then to the active FLASHRAM 528 and executable RAM 526. During operation, the software imagewithin the executable RAM 526 is executed on the CPU 520.

[0119]FIG. 15 shows a number of interfaces to an MCU 550, including anRS232 interface which may be connected to a craft terminal, a SONEToverhead connection 564, for receiving SDCC information from a SONETring connected to the device, an X.25 connection 566, a localcommunications network (LCN) interface 568, an Ethernet connection 570to which a craft PC may be connected, and an ATM virtual connection 572which may be a permanent virtual circuit (PVC). The ATM virtualconnection 572 is for example, one of a number of permanent virtualconnections provisioned to carry software image data or othermaintenance information between MCU 550 and MCUs of other networkelements.

[0120] During operation of the MCU 550, software image files may becommunicated to and from the MCU 550 using an FTP functional unit 556 inconjunction with a TCP/IP functional unit 558, via the ATM VC 572,Ethernet 570, or LCN 568. In addition, software image files may becommunicated with the MCU 550 using the OSI stack functional unit 560and FTAM functional unit 554 via SDCC over the SONET overhead connection564, or via the X.25 566, LCN 568, or Ethernet 570 interfaces. A commandinterpreter 552 operates to process Transaction Language 1 (TL1)commands received from a craft terminal over the RS232 connection 562.

[0121]FIG. 16 shows a “gateway” network element (GNE) 600, operationssupport station (OSS) 604, and a remote network element 602. The networkelement 600 is referred to as a “gateway”, for example, because itprovides a central management station such as the OSS 604, a connectionto a SONET ring to which the remote network element 602 is connected. Toinitiate a download of a software image from the gateway network element600 to the remote network element 602, a command is first issued by theOSS 604. Processing of the command results in an FTP request, whichcauses the FTP functional unit 610 to operate with the TCP/IP functionalunit 608 to form a number of IP packets containing the software image. Anumber of HDLC frames, which contain the IP packets storing the softwareimage, are then passed by the MCU 606 to the ATM switch fabric unit 614.The MCU 606 also passes a VCI/VPI value to the ATM switch fabric unit614. The VCI/VPI value indicates one of a number of provisioned PVCsthat are used to pass software image files from the gateway networkelement to remote network elements. In a first embodiment, each of theprovisioned PVCs connects the gateway network element with one remotenetwork element. In this way a star configuration is formed with thegateway network element as a hub. In a second embodiment, a PVC isprovisioned only to the next network element adjacent to the gatewaynetwork element. In this alternative embodiment, the software image fileis passed from adjacent network element to adjacent network element,until it arrives at an MCU having an IP address matching the destinationIP address of the IP packets carrying the software image file. In thisway, the software image file is passed from network element to networkelement. An IP termination and forwarding function 612 in the ATM unit614, extracts the ATM VCI/VPI value contained in the HDLC frames, andperforms segmentation of the file to form ATM cells, for example using asegmentation and reassembly (SAR) unit within the ATM switch fabric unit614. The IP termination and forwarding function 612 then passes the ATMcells to the switch fabric 616, which determines either that the VCI/VPIvalue in the cells indicates a permanent virtual circuit (PVC) 620between the GNE 600 and the remote NE 602 or, alternatively, between theGNE 600 and an adjacent network element, which may or may not be theremote NE 602. The ATM switch fabric 616 forwards the cells over the PVC620, by way of the line unit 618.

[0122] The cells are eventually received by the line unit 622 of theremote network element 602. The line unit 622 forwards the receivedcells to the switch fabric 626 within the ATM switch fabric unit 624.The switch fabric 626 recognizes that the PVC identified by the VCI/VPIvalue contained within the cells is terminated in the remote networkelement 602, and accordingly forwards the received cells to an IPtermination/forwarding function 628. The IP termination/forwardingfunction then employs a segmentation and reassembly unit to reassemblethe IP packets from the ATM cells, and forms a number of HDLC frames inwhich to forward the IP packets to the MCU 630. The MCU 630 thenexamines the IP destination address in the IP packets, and determinesthat the IP destination address is an IP address of the MCU 630. The FTPfunction then loads the software image file within the IP packets into amemory associated with the MCU 630.

[0123]FIG. 17 shows steps performed during downloading of an executablesoftware image from a gateway network element to a remote networkelement. As illustrated at step 650, a software download command isreceived by the MCU of the gateway network element. The download commandis for example, a TL1 copy-file command which indicates a source of thesoftware image file, as well as the destination network element to whichthe software image file is to be copied. The download command is forexample, received over an interface to the MCU such as an RS232connection to a craft terminal. Alternatively, the command may bereceived over an LCN, X.25, Ethernet connection to the MCU, or any othersuitable connection. The software image file to be downloaded may, forexample, be received within a number of IP packets, or, alternativelyover an X.25, LCN, or Ethernet interface to the MCU of the gatewaynetwork element.

[0124] The download command is processed by software executing in theMCU of the gateway network element, which issues an FTP Request to anFTP functional unit also within the MCU. The FTP functional unit uses aTCP/IP functional unit to form a number of IP packets having IPdestination addresses equal to an IP address of an MCU within the remotenetwork element, and containing the software image file indicated in thedownload command received at step 650. In an alternative embodiment, theIP destination address is a multicast address recognized by each devicewhich uses a common executable software image, and which is beingprovided a new executable software image by the download beingperformed.

[0125] Software within the MCU of the gateway network element thendetermines a VCI/VPI value of a permanent virtual circuit (PVC). In afirst embodiment, the PVC is between the gateway network element and theremote network element. In an alternative embodiment, the PVC is betweenthe gateway network element and an adjacent network element (which mayor may not be the remote network element in which the software image isto be loaded and executed). The MCU software then forms a number of HDLCframes containing the IP packets, as well as the VCI/VPI value of thePVC, and forwards these HDLC frames at step 654, over a serial point topoint connection to an ATM switch fabric unit within the local networkelement.

[0126] Upon receipt of the HDLC frames from the MCU, as illustrated atstep 656, software within the ATM switch fabric unit employs a SAR unit,also within the ATM switch fabric unit, to form a number of ATM cellshaving the VCI/VPI value provided from the MCU in their header. Thecells are then passed to an ATM switch fabric within the ATM switchfabric unit, which forwards the cells to an output interface of the ATMswitch fabric unit associated with the PVC. The selected outputinterface of the ATM switch fabric unit is, for example, coupled to aline unit within the local network element, which receives the cells,encapsulates them into an STS signal, and transmits them onto a SONETring.

[0127] As depicted at step 658, the remote network element, which is,for example, also coupled to the SONET ring, receives the STS signalcontaining the ATM cells storing the software image. The remote networkelement determines that the STS signal contains ATM cells. The remotenetwork element further determines that the STS signal is provisionedsuch that the ATM cells it contains are extracted and forwarded to anATM switch fabric unit within the remote network element.

[0128] The switch fabric within the ATM switch fabric unit, responsiveto the VCI/VPI value in the received cells, forwards the received cellsto an IP termination and forwarding functional unit within the ATMswitch fabric unit. The IP termination/forwarding unit uses a SAR unitwithin the ATM switch fabric unit to reassemble the IP packets from theATM cells. At step 660, the IP packets are then encapsulated into anumber of HDLC frames, which are forwarded to the MCU of the remotenetwork element.

[0129] At step 662, the MCU of the remote network element receives theHDLC frames containing the IP packets, and compares the destination IPaddress of those packets with an IP address of the MCU of the remotenetwork element. In response to a match, the MCU software extracts thesoftware image file within the IP packets and loads it into a memorywithin the MCU at step 664. In the alternative embodiment in which thePVC from the gateway network unit is with an adjacent network element,and where that adjacent network element is not the remote networkelement for which the software image is destined, there would not be amatch between the IP destination address of the packets and the IPaddress of the MCU of that adjacent network element. In that case, theMCU software of the adjacent network element would look up a VCI/VPIvalue associated with a PVC to an adjacent network element, and forwardthe packets back to the ATM switch fabric unit, along with the newVCI/VPI value. The ATM switch fabric unit would then form cells havingthe new VCI/VPI value in their headers, and forward the cells to anoutput interface associated with that PVC. The IP packets would then bereceived by the next adjacent network element, which would againdetermine if the destination IP address of the packets is the IP addressof an MCU within that network element. In this way the IP packetscontinue to be forwarded from network element to network element untilthey reach the target network element, into which the software image isdownloaded.

[0130] The functions herein described can be implemented in many forms,including one or more Application Specific Integrated Circuits or anyother suitable hardware implementation, or some combination of hardwarecomponents and software. Where a portion of the functionality isprovided using software, that software may be provided to the computerin many ways; including, but not limited to: (a) information permanentlystored on non-writable storage media (e.g. read only memory deviceswithin a computer such as ROM or CD-ROM disks readable by a computer I/Oattachment); (b) information alterably stored on writable storage media(e.g. floppy disks and hard drives); or (c) information conveyed to acomputer through communication media such as computer or telephonenetworks via a modem.

[0131] While the invention is described through the above exemplaryembodiments, it will be understood by those of ordinary skill in the artthat modification to and variation of the illustrated embodiments may bemade without departing from the inventive concepts herein disclosed.Accordingly, the invention should not be viewed as limited except by thescope and spirit of the appended claims.

1. A service unit for a network element, comprising: a first backplaneinterface to said network element, wherein said first backplaneinterface conveys payload data using a first communication protocol; anda second backplane interface to said network element, wherein saidsecond backplane interface conveys payload data using a secondcommunication protocol.
 2. The service unit of claim 1, wherein saidfirst communication protocol is asynchronous transfer mode.
 3. Theservice unit of claim 2, wherein said second communication protocol issynchronous transfer mode.
 4. The service unit of claim 1, wherein saidfirst communication protocol is Ethernet.
 5. The service unit of claim3, wherein said first backplane interface is to couple with a first starinterconnect configuration within said network element.
 6. The serviceunit of claim 5, wherein said second backplane interface is to couplewith a second star interconnect configuration within said networkelement.
 7. The service unit of claim 6, wherein said first starinterconnect includes a line unit slot as a hub, and wherein said secondstar interconnect includes a switch fabric slot as a hub, and whereinsaid first star interconnect further includes said switch fabric slot,and wherein said second star interconnect further includes said lineunit slot.
 8. The service unit of claim 1, further comprising a thirdbackplane interface to couple with a control star interconnectconfiguration within said network element.
 9. The service unit of claim1, further comprising an input output (I/O) interface external to saidnetwork element.
 10. The service unit of claim 9, wherein said inputoutput interface is a local area network interface.
 11. The service unitof claim 10, wherein said local area network interface is an Ethernetinterface.
 12. The service unit of claim 9, wherein said input outputinterface is a telecommunications interface.
 13. The service unit ofclaim 12, wherein said telecommunications interface is a digitalmultiplex hierarchy (DMH) interface.
 14. The service unit of claim 13,wherein said digital multiplex hierarchy interface is a digital signal 1(DS-1) interface.
 15. The service unit of claim 14, wherein said digitalmultiplex hierarchy interface is a digital signal 3 (DS-3) interface.16. The service unit of claim 12, wherein said telecommunicationsinterface is an optical carrier 3 (OC-3) interface.
 17. The service unitof claim 9, further comprising a data forwarding mechanism forforwarding a first data unit received from said first backplaneinterface to said second backplane interface, and a second data unitreceived from said first backplane interface to said input outputinterface.
 18. The service unit of claim 17, further comprising a dataforwarding mechanism for forwarding a first data unit received from saidsecond backplane interface to said first backplane interface, and asecond data unit received from said second backplane interface to saidinput output interface.
 19. The service unit of claim 9, furthercomprising a data forwarding mechanism for forwarding a first data unitreceived from said input output interface to said first backplaneinterface, and a second data unit received from said input outputinterface to said second backplane interface.